1. Field of the Invention
The present invention relates to a method and apparatus for abating pollutants in the exhaust gas streams of internal combustion engines. More specifically, the present invention provides a method and apparatus for abating pollutants in the exhaust gas streams during the cold-start period of engine operation.
2. Related Art
It is well known that the exhausts of internal combustion or other engines contain noxious pollutants including carbon monoxide ("CO"), unburned hydrocarbons ("HC") and nitrogen oxides ("NO.sub.x "). It is common practice to provide engine systems with catalysts designed to convert these noxious pollutants into relatively innocuous species such as carbon dioxide, water and nitrogen. The activity of such a catalyst varies directly with the temperature of catalyst operation, which, for catalysts used for gasoline-fueled automobile engines, is typically 300.degree. C. to 800.degree. C. Normally, the catalyst is heated to its operating temperature by exposure to the exhaust gases released by the engine and by exothermic reactions involved in the oxidation of CO and HC. However, it can take from about 90 to 120 seconds for a catalyst to be heated to operating temperature from ambient temperature, i.e, from a "cold start". During this warm-up period, sometimes herein referred to as "the cold-start period", an unacceptably high proportion of the noxious pollutants in the exhaust gas stream passes through the catalyst uneffected. To alleviate the discharge of these pollutants during the cold-start period, attempts have been made to place the catalyst, or a smaller, second catalyst, closer to the engine to avoid heat loss by the exhaust through the exhaust pipe, so that the catalyst is heated more quickly to its normal operation temperature.
U.S. Pat. No. 3,791,143 to Keith et al dated Feb. 12, 1974 discloses a process and apparatus for purifying exhaust gases of an internal combustion engine during a cold-start period. Two catalytic converters are placed in the exhaust gas line, an initial catalyst and a second catalyst. Supplemental fuel and air are added through a valved fuel line into the exhaust gas line upstream of the initial catalyst. The supplemental fuel may be a gas at ambient temperatures, but is preferably normally liquid, or may be a hydrocarbon boiling in the range of up to about 600.degree. F. at atmospheric pressure, or a mixture of hydrocarbons generically described as having from 3 to 12 carbon atoms (see column 7, lines 31-37). During the cold-start period, the mixture of supplemental fuel, exhaust gas and air flow into the initial catalyst which catalyzes the combustion of the fuel and releases heat (see column 9, lines 4-9). The second catalytic converter is heated by hot effluent gases coming from the initial catalytic converter (see column 5, lines 11- 13). When the second catalytic converter reaches its normal operating temperature, the initial catalytic converter may be bypassed, and the by-pass mechanism may be thermostatically controlled (see column 5, lines 38-45 and column 6, lines 5-9).
U.S. Pat. No. 3,779,015 to Maruoka dated Dec. 18, 1973 discloses an exhaust gas treatment system in which hot gases are generated by combusting an air/fuel mixture in a burner unit and are added to the exhaust gas line to bring the catalytic converter quickly up to its operating temperature. The apparatus may include thermostatic switches to control the supply of fuel in response to the temperature of the catalytic converter.
U.S. Pat. No. 4,359,863 to Virk et al dated Nov. 23, 1982 discloses an exhaust gas torch apparatus which comprises a catalytic segment through which a diesel exhaust gas stream flows before entering a diesel particulate trap. Periodically, the exhaust gas stream is preheated in an electrically powered heating zone, and is then passed into the catalytic segment to raise the catalyst to its "light-off" temperature. Supplemental fuel is then injected into the heated catalyst segment where it ignites upon contact with the catalyst. The combustion reaction travels into the particulate trap and oxidizes particulates trapped therein to regenerate the trap. The fuel may be propane or a suitable fluid such as diesel oil or kerosene (see column 5, lines 57-62).